Scroll down below the areas called "Intro" and "Extended
(Optional)" where you type in your diary. There is an area called
"Tags" with a box to enter tags into. "Add keywords that describe
this Diary." Add one or more key words in there that apply to your
diary, then click "save and preview." You can add more tags and
change tags after your diary is published, but you need at least
one to get it published.

(If you get that tag error message, you have to click "Edit" to
get back to the page to enter tags.)

This question comes up a lot. Here is another help desk thread
on it. I cut and pasted the answer from there.